Copper interconnect structure having stuffed diffusion barrier

ABSTRACT

The present invention provides a method of fabricating a semiconductor device, which could advance the commercialization of semiconductor devices with a copper interconnect. In a process of metal interconnect line fabrication, a TiN thin film combined with an Al intermediate layer is used as a diffusion barrier on trench or via walls. For the formation, Al is deposited on the TiN thin film followed by copper filling the trench. Al diffuses to TiN layer and reacts with oxygen or nitrogen, which will stuff grain boundaries efficiently, thereby blocking the diffusion of copper successfully.

REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to KoreanPatent Application Number 10-2000-0074025, filed Dec. 6, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to methods of fabricating asemiconductor device and, more particularly, to methods of forminginterconnect structures in a semiconductor device fabrication process.

2. Description of the Related Art

A process of fabricating a semiconductor integrated circuit is roughlydivided into the process of forming devices on a silicon substrate andthe process of electrically connecting the devices. The latter is calledan interconnection process or metallization, and is a key for improvingyield rate and reliability in the fabrication of semiconductor devicesas the devices become more highly integrated.

A metal widely used as an interconnection material is aluminum (Al).However, as the integration of devices is improved, the interconnectionline width is reduced, while its total length is increased, such thatthe signal transfer delay time, represented by the RC time constant, islengthened. Furthermore, the reduction in interconnection line widthresults in cutting in the interconnection line due to electromigrationor stress migration. Accordingly, in order to fabricate a reliabledevice with a fast operation speed, copper (Cu) is used instead ofaluminum (Al) for forming the interconnection lines since it has lowerresistance compared with that of Al and stronger resistance againstelectromigration and stress migration.

However, Cu lacks excellent properties that Al has other than the lowresistance and high melting point. For example, Cu cannot form a denseprotective layer such as Al₂O₃, has bad adhesive strength to SiO₂ and isdifficult to dry-etch. In addition, its diffusion coefficient in siliconis approximately 10⁶ times larger than that of Al, and it is known thatCu diffused into the silicon forms a deep level between band gaps.Furthermore, copper's diffusion coefficient in SiO₂ is known to belarge, which decreases the ability of SiO₂ to insulate between Cu lines.As a result of copper's large diffusion coefficient in silicon and SiO₂,the reliability of the semiconductor device is reduced. Accordingly, toensure the reliability of the device, a diffusion barrier capable ofpreventing Cu from rapidly diffusing into the silicon or SiO₂ isrequired.

Ta or TaN thin films deposited by a sputtering process are currentlybeing used as the diffusion barrier for Cu, instead of the TiN filmsgenerally used in the conventional Al interconnection line fabricatingprocess. However, when the diffusion barrier is deposited in a contactor trench structure using sputtering, step coverage deteriorates as thedevice size becomes smaller. This problem suggests that it would bebeneficial to form the diffusion barrier through a more conformalprocess, such as chemical vapor deposition (CVD).

However, the development of new processes for forming a reliablediffusion barrier for Cu requires a considerably long period of time andthis may delay commercialization of a semiconductor device employing aCu interconnect structure.

The present invention solves the above problems. It is an object of thepresent invention to provide methods for fabricating a semiconductordevice employing a diffusion barrier whose grain boundary is stuffedwith metal oxide, not a metal element constructing the diffusionbarrier. This may advance commercialization of a semiconductor devicewith a Cu interconnect structure.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a process for coppermetallization is provided. A diffusion barrier comprising grainboundaries is deposited over a semiconductor substrate and a layer ofreactive metal is deposited over the diffusion barrier. A differentmetal compound, such as an oxide or nitride of the reactive metal, isformed in the grain boundaries of the diffusion barrier. Copper is thendeposited over the diffusion barrier.

In one embodiment the diffusion barrier is a metal nitride, preferablyselected from the group consisting of titanium nitride, tungsten nitrideand tantalum nitride. More preferably the diffusion barrier is titaniumnitride. In a further embodiment the reactive metal is Al. In anotherembodiment the different metal compound is formed in the grainboundaries by annealing after deposition of the reactive metal layer.

In another aspect, the present invention provides a process for coppermetallization in which a metal nitride layer is deposited on asemiconductor substrate, a layer of reactive metal is deposited over themetal nitride layer, a second metal nitride layer is deposited over thereactive metal layer, and a metal compound is formed from the reactivemetal in the grain boundaries of the metal nitride layers. In apreferred embodiment, the metal nitride layers are titanium nitride andthe reactive metal is aluminum.

In a further aspect, the present invention provides a diffusion barrierfor a copper interconnect comprising a layer of metal nitride covered bya layer of reactive metal. The grain boundaries of the metal nitridelayer are stuffed with a different metal compound.

In one embodiment the metal nitride layer is titanium nitride, thereactive metal is aluminum, and the grain boundaries are stuffed withaluminum oxide. In another embodiment the reactive metal is silicon andthe grain boundaries of the metal nitride are stuffed with siliconoxide. In yet another embodiment the diffusion barrier additionallycomprises a second metal nitride layer over the layer of reactive metal.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention can be more fullyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings, in which:

FIGS. 1 to 6 are cross-sectional views showing a method of forming a Cuinterconnect structure according to an embodiment of the presentinvention.

FIG. 7 is a cross-sectional view of a Cu interconnect structureaccording to another embodiment of the present invention.

FIG. 8 is a graph showing the sheet resistances of samples based on thethickness of the Al thin film and the annealing temperature inexperimental Cu interconnect structures, comparing a conventionalstructure with structures formed according to one embodiment of thepresent invention.

FIGS. 9A to 9D are SEM photographs showing etch pits on a siliconsurface exposed after etching Cu, Al and TiN layers in experimental Cuinterconnect structures, comparing a conventional structure withstructures formed according to one embodiment of the present invention.

FIG. 10 illustrates a process for the growth of TiN/Al/TiN by ALD.

FIG. 11 shows a process for the growth of TiN/Al/Ti/TiN by ALD.

FIG. 12 illustrates a process for the growth of TiN by atomic layerdeposition (ALD) using plasma.

FIG. 13 illustrates a process for the growth of TiN by plasma enhancedALD (PEALD) using nitrogen radicals.

FIG. 14 shows a process for the growth of elemental aluminum usinghydrogen radicals.

FIG. 15 is a process for the ALD growth of elemental titanium withhydrogen radicals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present disclosure teaches processes for forming a diffusion barrierin the context of copper metallization, as well as diffusion barriersformed by these processes. However, the skilled artisan will appreciatethat the present processes and structures have applications in othercontexts.

The present invention will now be described in connection with preferredembodiments with reference to the accompanying drawings.

A diffusion barrier is a material that is inserted between twosubstances in order to prevent the substances from being mixing witheach other due to diffusion. In a semiconductor device fabricationprocess, the diffusion barrier is used not only to prevent the diffusionbetween a substrate and interconnection material but also to block theinterconnection material from diffusing into a dielectric film.

Diffusion barriers can be roughly classified into passive barriers,non-barriers, single crystal barriers, sacrificial barriers and stuffedbarriers. If the diffusion barrier remains thermodynamically stablebetween the interconnection material and substrate, it is a passivebarrier or non-barrier. The distinction between the two is based ondiffusion through the grain boundary of the diffusion barrier. That is,the diffusion barrier is a passive barrier when it there is littlediffusion through the grain boundary. On the other hand, a diffusionbarrier is a non-barrier, which does not serve as a diffusion barrier,when material diffuses easily through the grain boundary.

Diffusion barriers are sacrificial barriers if they arethermodynamically unstable such that there is a reaction with either theinterconnection material or the substrate. Thus, the sacrificial barrieritself reacts with the interconnection material or substrate material toprevent the diffusion of the material. The sacrificial barrier isconsumed according to the reaction so that it loses its function as thediffusion barrier when it has been completely consumed. When a diffusionbarrier cannot fulfill its function it is called “diffusion barrierfailure”. Thus, sacrificial diffusion barrier failure will occur after alapse of a predetermined time, however it performs its function untilthen.

Diffusion barrier failure in Cu interconnects is often the result of oneof the following three causes:

-   -   1) Diffusion of Cu or substrate atoms through defects in the        diffusion barrier, such as dislocations or vacancies;    -   2) Diffusion of Cu or substrate atoms through the grain boundary        of a polycrystalline diffusion barrier; and    -   3) Chemical reaction of the diffusion barrier with the Cu or        substrate material.

Failure of a thermodynamically stable diffusion barrier mainly dependson number 2 above, namely, the diffusion of Cu or substrate atomsthrough the grain boundary of a polycrystalline diffusion barrier. Thisis because the diffusion of the Cu or substrate atoms through the grainboundary occurs much more readily than diffusion through the grain.Accordingly, it is very important to prevent the diffusion through thegrain boundary.

There are several approaches to preventing diffusion through the grainboundary. First, forming the diffusion barrier using a single crystal oramorphous crystal having no grain boundary may avoid grain boundarydiffusion. The second approach is to block existing grain boundaries.Blocking the grain boundaries in a polycrystalline thin film is called‘stuffing’ and results in a ‘stuffed barrier.’

A method of “stuffing” the diffusion barrier that has been studied usesnitrogen stuffing and oxygen stuffing. Precipitates of nitrides oroxides have been formed at the grain boundaries of diffusion barriers bythe reaction of nitrogen or oxygen with metal elements present in thediffusion barriers. These precipitates stuff the grain boundaries of thediffusion barriers. In the case of TiN diffusion barriers, commonly usedin aluminum metallization, one method to improve the barrier propertiesis to stuff the grain boundaries with extra oxygen by annealing a TiNthin film deposited by PVD or CVD in a N₂ or O₂ ambient. For example,diffused oxygen in the grain boundaries of the TiN reacts to formtitanium oxide that is thought to stuff the grain boundaries.

The stuffing effect with nitrogen or oxygen works efficiently with Albut is not effective with Cu. Specifically, most of the oxygenintroduced into TiN films with the help of annealing diffuses throughthe grain boundaries of the TiN to oxidize the surface of the TiNgrains. The oxygen in the titanium oxide reacts easily with Al that hasdiffused through the grain boundaries to form Al₂O₃. However, in thecase of Cu, the enthalpy of formation of copper oxide is smaller thanthat of Ti oxide. Thus, Cu atoms that diffuse through the grain boundarydo not take oxygen from the titanium oxide and do not form copper oxide.The following Table 1 shows the enthalpy of formation of titanium oxide,aluminum oxide and copper oxide.

TABLE 1 Enthalpy of formation at 298K Type Phase (kJ/mol) Ti—O TiO −519.7 Ti₂O₃ −1521.6 Ti₃O₅ −2457.2 TiO₂  −944.0 Al—O Al₂O₃ −1675.7 Cu—OCuO  −168.6 Cu₂O  −157.3

As described above, a TiN thin film into which oxygen has beenincorporated through annealing can serve as an excellent stuffeddiffusion barrier for Al but is barely effective for Cu. Furthermore,because Cu is does not react strongly with nitrogen, it is difficult toimprove the property of the diffusion barrier by incorporatingimpurities.

Again, because the enthalpy of formation for copper oxide is smallerthan that of Ti oxide, Cu cannot form oxide at the grain boundary of theTiN. The following table 2 shows the tendency of oxide formationenthalpy for various metals.

TABLE 2 Ca V Nb Mo Hf Ta W Enthalpy of oxide CaO: V₂O₅: Nb₂O₅: MoO₃:HfO₂: Ta₂O₅: WO₃: formation [kJ/mol] −635 −1550 −1550 −745 −1144 −2045−842 Al Mg Ti Zr Cr Zn Be Enthalpy of oxide Al₂O₃: MgO: TiO₂: ZrO₂:Cr₂O₃: ZnO: BeO: formation [kJ/mol] −1656 −601 −944 −1097 −1139 −350−608

As shown in the table 2, metals such as Al, Zr, Cr, V, Nb, Hf and Tathat have an enthalpy of oxide formation larger than that of Ti oxidecan form oxides by reacting with oxygen atoms bound to Ti as theydiffuse along the TiN grain boundaries. Consequently, these metals canbe used to stuff the grain boundary.

A TiN thin film can serve as an effective diffusion barrier in Alinterconnection lines because oxygen contained in the TiN thin film cancombine with the Al to form an oxide that stuffs the grain boundaries.Thus, a metal whose enthalpy of oxide formation is lower than that ofthe metal element of the diffusion barrier can be used to “stuff” thegrain boundary of the diffusion barrier with the oxide of the metal,thereby forming a diffusion barrier that is effective for Cuinterconnection lines. Accordingly, in one embodiment of the presentinvention a compounding material such as oxygen is incorporated into thediffusion barrier in an amount sufficient for stuffing the grainboundary. In one arrangement, the oxygen is preferably incorporated intothe diffusion barrier before the stuffing step. Annealing is preferablycarried out to move the material through the grain boundary and tothereby form an oxide at the grain boundary.

In one aspect of the invention, a reactive metal element having a stronginclination to form an oxide is deposited or formed on the diffusionbarrier in the form of a thin film, such as by using a gas containingthe metal element or a solution including its ions. The diffusionbarrier thus contains a small amount of the metal element therein. Inthe case of the deposition of the metal element, the metal thin film isformed in minimum thickness such that the diffusion of the metal elementinto the Cu layer is restricted, while the resistance of the Cu layer isnot affected.

FIGS. 1 to 6 show an embodiment of a method of fabricating asemiconductor device according to the present invention. FIG. 1illustrates a part of the semiconductor device, including a substrate 10and a dielectric film 20 formed thereon. A plurality of elements may beformed on the semiconductor substrate 10, such as MOS transistors,bipolar junction transistors and resistors, for example. These elementshave been formed previously through fabrication processes executedbefore the illustrated step. The semiconductor device shown may employ amultilevel-interconnection structure. In this case, the substrate 10 caninclude the semiconductor elements and a metal layer that electricallyconnects the elements. A dielectric film 20 may be, for example, SiO₂,Si₃N₄, or a doped glass film. It is preferably formed through CVD orPECVD processes, depending on its composition. In a preferredembodiment, the dielectric film 20 is preferably formed of SiO₂deposited by CVD.

Next, as shown in FIG. 2, a via or trench 22 is formed in the dielectricfilm. The via or trench 22 may be formed using reactive ion etching anda mask that defines the boundary of the via or trench 22. In case of acontact hole through which a metal line comes into contact with anelement formed on the substrate or a lower metal line, the viapenetrates the dielectric film 20 to reach the substrate 10. In a fieldregion other than a contact hole, however, it does not reach thesubstrate 10 and instead forms a trench 22 in which a metal line is tobe formed. FIGS. 1 to 6 show the trench 22 formed at the field region.The skilled artisan will recognize this as a stage in damascenemetallization.

Referring to FIG. 3, a TiN thin film 32 is deposited by CVD on thedielectric film 20 having the trench 22 formed therein. The TiN thinfilm 32 is preferably formed to a thickness of approximately 100 Å.Thereafter, a process for incorporating a compounding material, such asoxygen or nitrogen, into the grain boundaries of the TiN thin film 32 isexecuted. Preferably, oxygen is incorporated into the grain boundary.This may be accomplished by exposing the TiN thin film 32 to the air, byannealing the TiN film in a furnace or by treating the TiN thin filmwith activated oxygen from an O₂-plasma.

Referring to FIG. 4, an Al thin film 34 serving as an intermediate metallayer is deposited on the TiN film 32, such as by CVD. In a preferredembodiment, the Al thin film 34 is formed to a thickness of about0.1-2.0 nm. The double layer 30 of the TiN thin film 32 and Al thin film34 serves as a new diffusion barrier after an annealing process.

Referring to FIG. 5, a Cu layer 40 is deposited on the double layer 30to fill the trench 22. The Cu layer 40 is preferably formed through PVD,electroplating or MOCVD. After the completion of the deposition of theCu layer, the surface of the semiconductor device is planarized as shownin FIG. 6. In a preferred embodiment, the planarization is carried outin such a manner that the TiN thin film 32, Al thin film 34 and Cu layer40 are nonselectively removed through chemical mechanical polishing(CMP). Note that CMP could be selective against a CMP shield over theinsulating layer 20. In another alternative embodiment, theplanarization may be performed by a nonselective plasma etching process.Upon completion of the planarization, a Cu interconnect 50 is exposedand the diffusion barrier 30, comprising the TiN thin film 32 and Althin film 34, is present between the dielectric film 20 and the Cuinterconnect 50. In the aforementioned process, an annealing process isperformed at least once following deposition of the Al film.

FIGS. 8 and 9 illustrate experimental examples of Cu interconnectstructures realized according to one embodiment of the presentinvention. In the examples shown, a TiN layer was deposited to thethickness of about 200 Å by pyrolytic deposition using the singleprecursor of TDMAT on an 8-inch silicon wafer. The wafer was cut intosamples with a size of 1 inch² and Al and Cu were continuously depositedthereon using DC magnetron sputtering. Annealing was carried out at apressure of below 5×10⁶ Torr in a vacuum ambient. The annealing wascarried out for one hour at temperatures from about 500-700° C., withthe temperature being increased in steps of about 50° C. The sheetresistance of each of the annealed samples was measured by using afour-point probe. FIG. 8 shows the measured result of the sheetresistance based on various thicknesses of the Al thin film anddifferent annealing temperatures. As shown in FIG. 8, the samples havingthe Al thin film deposited thereon at a thickness of 10 nm or greatereffectively block diffusion of Cu, compared to sample A, having only Cudeposited thereon.

The Cu layer, Al film and TiN film were removed using a chemicalsolution in order to estimate the diffusion barrier failure temperature,and then the silicon surface was Secco-etched. FIGS. 9A to 9D are SEM(Scanning Electron Microscopy) photographs of etch pits on the siliconsurface exposed by the etching. FIGS. 9A to 9D correspond to the foursamples (A, B, C and D) of FIG. 8, respectively, which were all annealedat 650° C. As shown in FIGS. 9A to 9D, the size and density of the etchpits are sharply reduced as the Al film thickness increases.

As a result of the estimation of the failure temperature, diffusionbarrier failure occurred in the sample having only the Cu layerdeposited thereon, without the Al film, after annealing for one hour at500° C. under vacuum. Diffusion barrier failure did not occur in thesamples with an Al film of 10 nm or greater thickness or more depositedthereon, even after annealing for one hour at 700° C. in the sameambient. From this result, it is understood that the CVD-TiN thin filmdeposited contains greater than 20 atomic % oxygen because its finestructure is porous enough that Al in contact with the TiN film diffusesinto the TiN grain boundaries during the annealing to form Al oxide atthe grain boundaries, thereby blocking Cu diffusion.

Although a specific embodiment has been illustrated and described above,it will be obvious to those skilled in the art that variousmodifications may be made. For example, although the Al thin film isformed as the intermediate metal layer on the single-level TiN diffusionbarrier 32 in the above-described embodiment, the TiN diffusion barriercan be formed in a multilevel structure. In the case of a multilevel TiNdiffusion barrier, the Al film is formed between the layers of themultilevel TiN diffusion barrier. FIG. 7 shows the case where the TiNdiffusion barrier is formed of two layers. Referring to FIG. 7, a firstTiN thin film 32-1 is deposited on the dielectric film 20, preferablyusing CVD, and then the process for stuffing the grain boundary thereofwith oxygen is carried out. Thereafter, Al is deposited on the first TiNthin film 32-1. As described above, the intermediate Al metal layer ispreferably deposited in the form of thin film on the first TiN film32-1, or an adhesion layer is formed on the TiN film using a gascontaining Al or a solution having Al ions.

Subsequently, a second TiN thin film 32-2 is deposited on theintermediate metal layer using CVD, thereby forming a multilevel TiNdiffusion barrier. The Cu layer 40 is deposited on the second TiN film32-2 through the aforementioned process. After the deposition of thesecond TiN thin film 32-2, annealing is preferably carried out for thedeposited structure at least once. With the structure shown in FIG. 7,Al diffuses into the first and second TiN thin films 32-1, 32-2 duringthe annealing and combines with O₂ existing in the films to form Aloxide at the grain boundaries of the first and second TiN thin films32-1, 32-2, thereby effectively blocking the diffusion of Cu. Inaddition, a further intermediate metal layer can be formed on the secondTiN thin film 32-2 to block the diffusion of Cu more effectively.

Although Al is used for the intermediate metal layer in the aboveillustrated embodiment of the invention, Zr, Cr, V, Nb, Hf or Ta, whichhave a stronger inclination to formation oxide than Ti, can also be usedas a material for forming the intermediate metal layer as shown in Table2. In other words, the intermediate metal layer is formed of a metalelement that forms an oxide thereof more easily than the metal elementof which the diffusion barrier is comprised.

Furthermore, the intermediate metal layer can be formed through a methodother than CVD, such as PVD, electroplating, electrodeless plating, wetchemical contamination and atomic layer deposition (ALD).

Although the diffusion barrier is formed of TiN in the aforementionedembodiment, TaN or WN can be also used for the diffusion barrier anddeposited using CVD. In addition, the above-described embodiment relatesto the fabrication of a Cu interconnect structure located on thedielectric film. However, in the actual semiconductor device fabricationprocess, a contact hole for connecting the metal line forinterconnection to an element formed on the substrate or a lower metalline can be formed. In the structure having this contact hole, thediffusion barrier is deposited on the substrate or lower metal linelocated under the contact hole, the intermediate metal layer is formedon the diffusion barrier and the Cu layer is deposited on theintermediate metal layer. In a further variation of this embodiment ofthe invention, the Cu layer is not deposited in the contact hole, ohmiccontact is made using only the intermediate metal layer, and the Cuinterconnect structure is employed only in the field area.

As described above, a novel diffusion barrier combines a layercomprising a diffusion barrier layer used in the conventional Alinterconnect structure, for example, a TiN film, with an intermediatemetal ultra thin film, preferably comprised of a metal selected from thegroup consisting of Al, Zr, Cr, V Nb, Hf and Ta. By forming thediffusion barrier for the copper line in an easier way, thecommercialization of copper as the line material can be advanced.

In another embodiment, a diffusion barrier comprising grain boundariesis deposited on a semiconductor substrate. Preferably the diffusionbarrier comprises a metal nitride, such as titanium nitride, tungstennitride or tantalum nitride. More preferably the diffusion barrier istitanium nitride. A thin layer of a reactive metal is deposited on thediffusion barrier layer. The reactive metal reacts with a compoundingmaterial, particularly nitrogen or oxygen, to form a different metalcompound that expands within and stuffs the grain boundaries of themetal nitride layer. Preferably the reactive metal reacts with oxygen toform a metal oxide during an annealing process. The oxygen may have beenincorporated in the metal nitride layer prior to deposition of thereactive metal layer. Alternatively the oxygen may be supplied duringthe annealing.

In a further embodiment, a diffusion barrier comprising grain boundariesis deposited on a semiconductor substrate. Preferably the diffusionbarrier comprises a metal nitride, such as titanium nitride, tungstennitride or tantalum nitride. More preferably the diffusion barrier istitanium nitride. A thin layer of a reactive metal is deposited on thediffusion barrier layer. A second layer of metal nitride is thendeposited on top of the reactive metal layer, to form a laminatestructure.

FIG. 10 illustrates a process for depositing a TiN/Al/TiN structure. A 5nm layer of TiN is grown 170 by ALD on a semiconductor substrate. Next,a 2 nm aluminum layer is grown 172 by ALD on the TiN layer. Finally a 5nm layer of TiN is grown 174 by ALD on the aluminum layer.

In yet a further embodiment an additional metal layer is deposited.Thus, a diffusion barrier comprising grain boundaries is deposited on asemiconductor substrate. The diffusion barrier is preferably a metalnitride, such as TiN. A first reactive metal layer, such as an aluminumlayer, is deposited thereon. Next, a second metal layer is deposited onthe first reactive metal layer. A second diffusion barrier, such as ametal nitride layer, is then deposited on the second metal layer.According to one embodiment the second metal layer is a titanium metallayer that covers the surface of the first reactive metal and protectsit against the nitrogen source chemical that is used for the growth of asecond diffusion barrier layer, such as a metal nitride layer. A processfor producing structures according to this embodiment is diagramed inFIG. 11.

As shown in FIG. 11, a 5-nm layer of TiN is grown 190 by, e.g., ALD on asubstrate surface. Next, a 2-nm aluminum layer is grown 192 by, e.g.,ALD on the TiN surface. Then a 1-nm layer of Ti is grown 194 by, e.g.,ALD on the aluminum surface. The Ti layer protects the aluminum layeragainst the nitrogen source chemical that is used to grow the secondmetal nitride layer. Finally, a 4-nm layer of TiN is grown 196 by, e.g.,ALD on the titanium surface. Because ALD often utilizes highly reactivesource chemicals, in the beginning of the TiN layer growth 196, thenitrogen source chemical might react with the titanium metal (depositedat step 194) and convert the titanium metal into titanium nitride. Inthat case the resulting thin film structure would be TiN/Al/TiN, insteadof TiN/Al/Ti/TiN.

In each of the embodiments described above, the diffusion barriercomprising grain boundaries may be treated with oxygen or nitrogen, forcompounding with the reactive metal in the grain boundaries, prior todeposition of the reactive metal layer. Such treatment may comprise, forexample, exposure to oxygen in the atmosphere, an oxygen anneal processor treatment with O₂ plasma. As a result, oxygen or nitrogen isincorporated in the grain boundaries of the diffusion barrier. Thecompounding material may be present in an unreacted form, such as freeoxygen or nitrogen, and/or may have previously reacted with the metal ofthe diffusion barrier at the grain boundaries forming, e.g., TiO₂. Atany point following deposition of the reactive metal layer, an annealstep is carried out such that the reactive metal reacts with the oxygenor nitrogen present in the grain boundaries of the metal nitride layerto form an oxide or nitride stuffing the grain boundaries. This metaloxide or nitride incorporates the reactive metal, which is differentfrom the metal in the metal nitride.

Alternatively, an anneal process is carried out in the presence ofoxygen after deposition of the reactive metal layer. In the case of thelaminate embodiments, the oxygen anneal may be carried out immediatelyfollowing deposition of the first reactive metal layer, or after thedeposition of a subsequent reactive metal and/or further metal nitridelayer. The reactive metal layer reacts with oxygen during the oxygenanneal to form a metal oxide that stuffs the grain boundaries of thediffusion barrier.

The term “reactive metal” or “elemental metal” as used herein, broadlyrefers to any element that can react with oxygen to form an oxide ornitrogen to form a nitride and stuff the grain boundaries of a diffusionbarrier layer, particularly a metal nitride layer. Reactive metalsinclude, but are not limited to, aluminum, silicon, titanium, metals ingroups IVB, VB and VIB of the periodic table, such as Zr, Hf, V, Nb, Ta,Cr, Mo and W, germanium, magnesium, yttrium, lanthanum, and metals inGroup III of the periodic table, including the lanthanide series (e.g.,Sc, Ce, pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu).

Metal nitride layers, for example TiN layers, may be formed by anymethod known in the art, including PVD, CVD, ALD and PEALD. Preferably,damascene trenches and/or contact vias are lined with metal nitride byALD for forming conformal layers.

In one embodiment a TiN layer is preferably formed by atomic layerdeposition (ALD), more preferably by plasma enhanced atomic layerdeposition (PEALD). FIG. 12 is a flowchart illustrating an exemplaryPEALD process for depositing a TiN layer. A titanium source chemical ispulsed 100 to the reaction chamber. Titanium source chemical moleculesadsorb on the substrate surface, self-limitingly, to form no more than amonolayer. Surplus titanium source chemical molecules and possiblereaction byproducts are purged away 102. Hydrogen radicals are formedand contacted with the substrate surface for a pulse time 104. Theadsorbed titanium source chemical molecules are reduced into elementaltitanium metal atoms by the hydrogen radicals. Excess hydrogen radicalsand reaction byproducts are purged away 106. A nitrogen source chemicalis pulsed into the reaction chamber 108. Excess nitrogen source chemicalmolecules are purged away 110 and the cycle 112 is repeated until alayer of TiN with the desired thickness is grown. A typical thicknessfor the TiN layer is 5 nm to 10 nm.

FIG. 13 illustrates another possible process for depositing a TiN layerby PEALD. A titanium source chemical is pulsed 100 to the reactionchamber. Titanium source chemical molecules adsorb on the substratesurface, self-limitingly, to form no more than a monolayer. Surplustitanium source chemical molecules and possible reaction byproducts arepurged away 102. Nitrogen radicals are formed and contacted with thesubstrate surface for a pulse time 120. Several exemplary nitrogensource chemicals that may be used in forming metal nitrides includeradicals of N, NH and NH₂ as well as excited N₂*. The adsorbed titaniumsource chemical molecules react with nitrogen radicals and form titaniumnitride on the surface. Excess hydrogen radicals and reaction byproductsare purged away 122. The cycle 124 is repeated until a layer of TiN ofthe desired thickness is grown. A typical thickness for the TiN layer is5-10 nm.

The reactive metal layer may also be deposited by any method known inthe art, such as by PVD, CVD, ALD or plasma enhanced ALD (PEALD)processes. In one embodiment the metal layer is deposited by ALD. Sourcechemicals for use in ALD of the reactive metal layer include, forexample, alanes, alkyl aluminum, silanes, germanes, alkylamido titaniumcompounds such as tetrakis(ethylmethylamido) titanium, andcyclopentadienyl metal compounds.

In the case of a thin film of a very reactive metal, such as aluminum,an ALD process preferably utilizes pulsed plasma and inactive purgingand carrier gases. Inactive gases are preferably selected from the groupconsisting of argon (Ar), helium (He), hydrogen (H₂) and mixturesthereof.

For example, in one embodiment a layer of elemental aluminum isdeposited by PEALD as outlined in FIG. 14. An aluminum source chemicalis pulsed 130 into the reaction chamber. Exemplary ALD source chemicalsfor the deposition of aluminum include alanes and alkyl aluminum (e.g.,trimethyl aluminum). A preferred aluminum source chemical isdimethylethylamine alane (DMEAA), which lacks aluminum-carbon bonds andis therefore less likely to lead to carbon contamination of the aluminumlayer. When DMEAA is used, the temperature is preferably kept below thethermal decomposition limit, normally less than about 120° C., morepreferably less than about 100° C., and above the condensation limit.Aluminum source chemical molecules adsorb on the substrate surface.Excess aluminum source-chemical molecules and possible reactionbyproducts are purged away 132. Hydrogen radicals are formed andcontacted with the substrate surface for a pulse time 134. The adsorbedaluminum source chemical molecules react with hydrogen radicals and formelemental aluminum atoms on the substrate surface. Excess hydrogenradicals and reaction byproducts are purged away 136 and the cycle 138is repeated until a layer of elemental aluminum of the desired thicknessis grown. A typical thickness for an aluminum metal layer is 2 nm.

In another embodiment a layer of elemental aluminum is deposited by CVD.Deposition of an aluminum layer by CVD is described, for example, in Kimet al. (“Microstructure and deposition rate of aluminum thin films fromchemical vapor deposition with dimethylethylamine alane,” Appl. Phys.Lett. 68 (25): 3567-3569 (1996)) and in Li et al. (“Structuralcharacterization of aluminum films deposited on sputtered-titaniumnitride/silicon substrate by metalorganic chemical vapor deposition fromdimethyethylamine alane,” Appl. Phys. Lett. 67(23): 3426-3428 (1995)),both of which are hereby incorporated by reference. In both processes, apreferred aluminum source chemical is DMEAA.

A layer of elemental titanium can also be deposited by ALD, asillustrated in FIG. 15. A titanium source chemical is pulsed 150 intothe reaction chamber. Titanium source chemical molecules adsorb on thesubstrate surface, self-limitingly, leaving no more than a monolayer.Excess titanium source-chemical molecules and possible reactionbyproducts are purged away 152. Hydrogen radicals are formed andcontacted with the substrate surface for a pulse time 154. The adsorbedtitanium source chemical molecules react with hydrogen radicals and formelemental titanium atoms on the substrate surface. Excess hydrogenradicals and reaction byproducts are purged away 156. The cycle 158 isrepeated until a layer of elemental titanium is grown. A typicalthickness for the titanium metal layer is 0.5-2 nm.

In yet another embodiment a stuffed diffusion barrier is formed bydepositing a first TiN layer by ALD, depositing an aluminum layer by ALDor CVD on the first TiN layer and depositing a second TiN layer by ALDon the aluminum layer. The structure is annealed in the presence ofoxygen, either after depositing the aluminum layer or after depositingthe second TiN layer. The aluminum reacts with oxygen during the oxygenanneal to form Al₂O₃ that stuffs the nitride grain boundaries of one orboth TiN layers.

In a further embodiment a stuffed diffusion barrier is formed bydepositing a layer of TiN, depositing a layer of silicon thereon,depositing an optional further TiN layer thereover, and annealing suchthat the silicon reacts with oxygen to form silicon oxide that stuffsthe grain boundaries of the TiN layer.

Although the foregoing invention has been described in terms of certainpreferred embodiments, other embodiments will be apparent to those ofordinary skill in the art, in view of the disclosure herein.Accordingly, the present invention is not intended to be limited by therecitation of the preferred embodiments, but is instead to be defined byreference to the appended claims.

1. A diffusion barrier for a copper interconnect comprising a firstlayer of metal nitride directly contacting and covered by a layer ofreactive metal, and a second layer of metal nitride directly contactingand over the layer of reactive metal, wherein the grain boundaries ofthe first and second metal nitride layers are stuffed with a metalcompound of the reactive metal, wherein the reactive metal is adifferent metal from each metal in the metal nitride layers and isselected from the group consisting of Al, Si, Ti, Zr, Hf, V, Nb, Ta, Cr,Mo, W, Mg, Y and La, and wherein the diffusion barrier directlyunderlies a copper layer of the copper interconnect.
 2. The diffusionbarrier of claim 1, wherein the metal nitride of the first metal nitridelayer is selected from the group consisting of titanium nitride,tungsten nitride and tantalum nitride.
 3. The diffusion barrier of claim2, wherein the metal nitride of the first metal nitride layer istitanium nitride.
 4. The diffusion barrier of claim 1, wherein thereactive metal is Al.
 5. The diffusion barrier of claim 1, wherein thereactive metal is Si.
 6. The diffusion barrier of claim 1, wherein themetal compound is an oxide of the reactive metal.
 7. The diffusionbarrier of claim 6, wherein the metal compound is selected from thegroup consisting of aluminum oxide and silicon oxide.
 8. The diffusionbarrier of claim 1, wherein the metal compound is a nitride of thereactive metal.
 9. The diffusion barrier of claim 8, wherein the metalcompound is selected from the group consisting of aluminum nitride andsilicon nitride.
 10. The diffusion barrier of claim 1, wherein the firstmetal nitride layer is about 5 to 10 nm thick.
 11. The diffusion barrierof claim 1, wherein the reactive metal layer is about 2 nm thick. 12.The diffusion barrier of claim 1, wherein the reactive metal is alanthanide.
 13. A diffusion barrier for a copper interconnectcomprising: a first layer of metal nitride; a layer of reactive metaldirectly contacting and over the first layer of metal nitride whereinthe reactive metal is selected from the group consisting of metals ofgroup IIIB of the periodic table, metals of group IVB of the periodictable, metals of group VB of the periodic table and metals of group VIBof the periodic table; and a second layer of metal nitride directlycontacting and over the layer of reactive metal, wherein the grainboundaries of the first and second metal nitride layers are stuffed witha compound of a metal different from the metal in the nitride layers andthe second layer of metal nitride underlies and contacts a copper layerof the copper interconnect.
 14. The diffusion barrier of claim 13,wherein the compound of a metal different from the metal in the nitridelayers is selected from the group consisting of an oxide of the reactivemetal and a nitride of the reactive metal.
 15. The diffusion barrier ofclaim 1, wherein the first layer of titanium nitride is deposited byatomic layer deposition (ALD).
 16. A diffusion barrier for a copperinterconnect that directly contacts and underlies a copper filler of thecopper interconnect, the diffusion barrier comprising a first layer oftitanium nitride directly contacting and covered by a layer of aluminumand a second layer of titanium nitride between the aluminum layer andthe copper filler, wherein the grain boundaries of the first titaniumnitride layer are stuffed with aluminum oxide.
 17. The diffusion barrierof claim 1, wherein the layer of metal nitride comprises titaniumnitride.
 18. A diffusion barrier for a copper interconnect comprising alayer of metal nitride directly contacting and covered by a layer ofsilicon and a second layer of metal nitride directly contacting and overthe layer of silicon, wherein the grain boundaries of the metal nitridelayer are stuffed with silicon oxide and the diffusion barrier directlyunderlies a copper layer of the copper interconnect.
 19. The diffusionbarrier of claim 18, wherein the second layer of metal nitride comprisestitanium nitride.